In a small bowl, whisk together flour, baking soda and salt until well combined; set aside.

In a medium bowl, whisk together the melted butter and both sugars until well combined and smooth, about 30 seconds. Add the vanilla and egg; whisk until smooth, about 30 seconds. Add the mashed banana and whisk to combine.

Add the flour mixture to the butter mixture and use a spoon or rubber spatula to stir until just combined (dough will be soft). Stir in the walnuts and chocolate chips.